1. Field of the Invention
This invention relates generally to the manufacture of glass sheet and, more particularly, to glass sheet used for the production of TFT/LCD display devices that are widely used for computer displays.
2. Description of Related Art
The glass that is used for semiconductor powered display applications must have very high surface quality to allow the successful application of semiconductor type material. Sheet glass made using the apparatus of U.S. Pat. No. 3,338,696 assigned to Coming, Inc. makes the highest quality glass as formed and does not require post-processing. The Coming patent makes glass by a manufacturing process termed “The Overflow Process”. Glass made using other processes requires grinding and/or polishing and thus does not have as fine a surface finish. The glass sheet must also conform to stringent thickness variation and warp specifications.
FIGS. 1A through 1D illustrate the principle parts of a typical “Overflow Process” manufacturing system. The molten glass (2) from the melting furnace and forehearth, which must be of substantially uniform temperature and chemical composition, enters the forming apparatus from the downcomer pipe (7) at the downcomer pipe bottom end (17) into the inflow pipe (8) (also called an inlet pipe) and flows into the sheet forming block (1). The glass sheet forming apparatus, which is described in detail in U.S. Pat. Nos. 3,338,696 and 6,748,765 and in the patent application Ser. No. 10/214,904, includes a shallow trough on the top of a wedge shaped forming block (1). Straight sloped weirs (4) substantially parallel with the pointed edge of the wedge, herein termed the root (5), form each side of the trough in the forming block (1).
The trough bottom (6) and the sides of the trough are contoured in a manner to provide even distribution of the glass (2) to the top of each side weir (4). The molten glass (2) then flows through the trough, over the top of each side weir (4), down each side of the wedge shaped sheet forming block (1), and joins at the root (5) to form a sheet of molten glass. The molten glass is then cooled as it is pulled off the root (5) to form a solid glass sheet (10) of substantially uniform thickness.
The refractory materials from which the forming block and its support structure are made have high strength in compression and low strength in tension. Like most structural materials they also change shape when stressed at high temperature by a process termed thermal creep.
FIGS. 2A through 2D illustrate the typical effects of thermal creep on the shape of the forming block when the end support and compression blocks impart different compression stress to the bottom of the forming block (1) near the root (5). FIG. 2A shows that with no compression loading the forming block (1) sags in the middle such that the top of the weirs (4) and the root (5) are now curved (21) and the trough bottom (6) has a change in curvature (21). This curvature (21) causes the molten glass (2) to no longer flow with constant thickness (22) over the weirs (4). More specifically, the curvature (21) allows more glass to flow over the middle of the weirs resulting in an uneven sheet thickness distribution. The forming block (1) has an initial length (20) as defined by the phantom lines (24) and (29). With no external loading the weirs (4) get shorter and the root (5) gets longer.
FIG. 2B shows that sagging of the forming block is minimized under the optimum compression loading (26) of the lower section of the forming block (1) near the root (5). With optimal loading both the weirs (4) and the root (5) shorten equally to length (27). FIG. 2C shows that if too much load (25) is applied to the lower section of the trough (1) near the root (5), the root (5) is compressed excessively, thus producing a convex shape (23) to the trough weirs (4), the trough bottom (6), and the root (5). The root (5) shortens considerably more than the weirs (4) as can be seen by the movement relative to the phantom lines (24) and (29). FIGS. 2A through 2C represent the effect of thermal creep over the same time period. FIG. 2D shows a forming block (1), which has shortened a greater amount to length (28). This increased shortening is caused by imparting the correct load (26) for the increased time of a substantially longer production campaign. This increased shortening has an adverse effect on the width of the manufactured sheet.
The application of the optimum compression loading (26) minimizes the sagging of the forming block (1), but it does not maintain an optimum shape of the forming block. The flow distribution of the glass flowing in the forming block (1) is greatly improved, however, there is still a measurable amount of distortion of the forming block (1). FIG. 28 shows a typical forming block (281) magnified relative to its original shape (1), when acted upon by the optimum compression load (26). The forming block weirs (284) and trough bottom (286) have a slightly distorted curved shape.
U.S. Pat. No. 3,451,798 teaches that a sheet glass edge control device, termed “edge director” herein, must be installed at each end of the trough to prevent narrowing of the formed sheet as a result of surface tension. FIG. 3A through 3D show the prior art edge director assemblies (41) and (42), shown in FIGS. 4A through 4F, attached to the ends of the trough forming block (1). The flanges (47) of the inflow edge director assembly (41) are compressed against the forming block (1) by the inflow support and compression block (31). The inflow support and compression block (31) rests on the inflow end support structure (33) and is held in position by the adjustment device (34). The flanges (48) of the far end edge director assembly (42) are compressed against the forming block (1) by the far end support and compression block (32). The far end support and compression block (32) rests on the far end structure (35) and is held in position by the force motor (38). A force motor (38) is a device that generates a substantially constant linear force and may be physically implemented as either an air cylinder, a hydraulic cylinder, a spring assembly, an electric motor, or a weight and lever system.
As illustrated, the glass attaches to the fences (43) and (44) of the edge director assemblies (41) and (42) and maintains the same width as it flows down the sides of the forming block (1) from the weirs (4) to the root (5). The glass narrows under the influence of surface tension only as it leaves the bottom of the fences (43) and (44). The force motor (38) applies the compression force (36) to the forming block (1), which is restrained at the inflow end by the adjustment device (34). Over time thermal creep caused by the applied load (36) forces the edge director assemblies (41) and (42) to move closer to each other, thus producing a narrower sheet.
FIGS. 4A through 4C are side, end, and top views of the inflow end edge director (41) as used in the prior art. The inflow end edge director (41) has a fence (43) to which the glass attaches such that the width is maintained. The edge director (41) also has symmetrical edge director surfaces (45) that provide for gravity to assist the flowing glass to attach to the fence and flanges (47) that are used to secure the edge director to the inflow end of the forming block (1).
FIGS. 4D through 4F are side, end, and top views of the far end edge director (42) as used in the prior art. The far end edge director (42) has a fence (44) to which the glass attaches such that the width is maintained. The edge director (42) also has symmetrical edge director surfaces (46) that provide for gravity to assist the flowing glass to attach to the fence and flanges (48) that are used to secure the edge director to the far end of the forming block (1). Attached to the outlet edge director (42) is a wedge shaped protrusion, herein termed a plow (49), which aids in the control of the glass flow over the weirs (4) near the far end edge director (42).
The edge directors are normally fabricated via welding from platinum or platinum alloy sheet (platinum herein). In the prior art, the edge directors are fixed to each end of the forming block. Thus, as the campaign progresses and the forming block becomes shorter via thermal creep, the manufactured sheet becomes narrower. This results in less square feet of production and required process adjustments.
A major drawback of the apparatus of “The Overflow Process” is that the forming apparatus deforms during the manufacturing campaign in a manner such that the glass sheet no longer meets the width specification. This is a cause for premature termination of the production run.
Another major drawback of the apparatus of “The Overflow Process” is that there is no means for adjusting the width of the sheet which is manufactured.
Another major drawback is that although the compression loading of the bottom ends of the forming block corrects for the major portion of the forming block sag caused by thermal creep, this compression loading does introduce a measure of distortion of the forming block weirs and trough bottom.
Therefore, there is a need in the art for an apparatus, which maintains a constant glass width, and is capable of adjusting the width of the sheet. There is also a need in the art for an apparatus, which has additional means to control the shape of the forming block weirs and trough bottom when the forming block deforms due to thermal creep.